The present invention is directed to a rotary tubular screen which includes mechanism for imparting a vibratory action to the screen. The mechanism is so arranged that the vibratory action is applied to the screen only when the combined weight of the screen and material contained in the screen exceeds a preselected minimum weight, thus preventing the application of an excessive vibratory force to the screen. Because the vibratory force applied is most useful when a screen of this type is substantially full of material to be separated, application of the same vibratory force to the screen when the screen is substantially empty can cause an excessive vibratory motion of the screen.
In accordance with the present invention, the vibratory mechanism may consist of a simple flattened roller frictionally driven by the screen, but capable of contacting the screen only when a sufficient amount of material is contained in the screen to adequately absorb the vibratory forces applied.